The concept of providing a central lighting source and channeling the light output therefrom to various remote locations using optical fibers, light guides or the like has been proposed for various applications including automotive, display lighting and home lighting. An example of a central lighting scheme for an automotive application can be found in U.S. Pat. No. 4,958,263 issued to Davenport et al. on Sep. 18, 1990 which is assigned to the same assignee as the present invention. The goal of this automotive central lighting scheme as well as any other central lighting scheme, is to achieve the most efficient light output at the point of light delivery and to deliver such light output in a manner that allows for the specific lighting design considerations. For instance, in an automotive lighting design application, recent concerns have been towards improving the aerodynamic properties of the vehicle front end by reducing the space needed to accommodate forward lighting. As such, it would be advantageous if the designer could provide the necessary forward lighting using a design space on the order of approximately two inches in height. It is known however that to achieve such a design constraint and still provide the necessary illumination pattern, a small narrow beam of light is needed from the output end of the optical fiber. For example, in order to provide good illumination and beam control from a two inch high headlamp, it is necessary to utilize an optical fiber having a cross-sectional dimension of approximately 6 to 8 mm and to deliver from such optical fiber, at least 500 lumens (per headlamp) into f/1 optics. Additionally, in order to provide for the use of this small dimension optical fiber, it is necessary to provide a light source having an arc gap substantially less than the 4 mm arc gap typically used for automotive headlamps. This size limit requirement is due to the fact that when a typical elliptical reflector is used to focus the light from the arc onto the entrance face of the optical fiber, the reflector will magnify the arc gap length by a factor of between three and four times. As a side benefit of providing this controlled beam output, the designer achieves cost, size, weight and design flexibility benefits by use of the smaller diameter optical fibers.
Of further importance to the designer of lighting systems using a centralized light source and a small diameter light transmission medium to deliver light output remotely, is the fact that the brightness of the light source must be at a relatively high level. The photometric definition for brightness (more precisely, luminance) is the number of lumens per unit area per unit solid angle. The usual device for directing light from the discharge arc into the optical fiber or light guide is an elliptical reflector with the arc at one focus and the input face of the optical fibers at the second focus. In this arrangement, the brightness (luminance) at the fiber is proportional to the arc lumens divided by the gap.sup.2. It is useful to define arc lumens divided by gap.sup.2 as the effective brightness of the arc. For example, it has been determined experimentally that superior headlamp illumination and beam control is obtained by coupling 1000 lumens to each headlamp through an optimized optical collector and light guide at 55% efficiency, from a 2.7 mm long arc gap. The effective brightness of the arc to provide superior beam performance would therefore be: ##EQU1##
The light source disclosed in the above-discussed centralized automotive lighting patent achieves an effective brightness so defined, on the order of 34,000 lumens per cm.sup.2. This effective brightness level is accomplished by use of the discharge arctube light source described in U.S. Pat. No. 4,968,916 which issued to Davenport et al on Nov. 6, 1990 and is assigned to the same assignee as the present invention. This light source has a pressurized gas fill consisting of a metal halide, an amount of mercury in the range of between 5 and 50 mg per cubic centimeter of bulb volume and an inert gas having a pressure in the range of between 10 Torr and 15,000 Torr. U.S. Pat. No. 4,968,916 further discloses that the light source can have a cylindrical, ellipsoidal or tubular shape with the general dimensions of: a length in the range of 5 mm to about 100 mm, a central portion with a diameter of about 4 mm to 25 mm, a volumetric capacity of about 0.1 to 30 cubic centimeters and a predetermined distance, or arc gap between the electrodes of between 1 and 5 mm.
In actual practice, it is known that arc gaps for the typical metal halide discharge light source must be on the order of at least 4 mm so as to operate at advantageously high arc voltages in a sufficiently low density range to be free of convective instability. In fact, if one were to utilize an arc gap less than the typical 4 mm value for the lamp disclosed in U.S. Pat. No. 4,968,916 and still maintain an operating voltage which yields an acceptable efficacy value, it would be necessary to increase the mercury density in this lamp to a value significantly higher than this design contemplates. For a discharge lamp, mercury density, that is, the amount of mercury per volume, is an important design consideration for several reasons. By the known relationship between the operating voltage and the product of the arc gap and approximately the square root of the mercury density (see equation (1) below), it can be seen that a decrease in the arc gap below the 4 mm value typically practiced, must be accompanied by an exponential increase in the mercury density in order to maintain the necessary operating voltage. Such an increase in mercury density adversely affects other lamp operation characteristics however such as convective stability and stress on the material from which the arc tube is constructed. Of course, it is known that convective stability is dependent upon the dimensions of the arctube as well as the fill density and that to increase the fill density and the arctube diameter without limit, a risk of convective instability arises. It is a further challenge to the convective stability of the arc, and the mechanical integrity of the arc tube when a cold-fill pressure of several atmospheres of Xenon is added to provide for instant light warm-ups. Accordingly, it would be advantageous if one were to develop a discharge lamp having a shorter arc gap that achieves a high brightness level, particularly one on the order of approximately 2.5-3.0 mm with a brightness level in excess of 50,000 lumens per cm.sup.2 and wherein such short arc gap discharge lamp could operate at the higher pressures without risk of failure or damage to the integrity of the arc tube in which the discharge occurs, and without the risk of convective instability that would cause flicker in the light output. Effective brightness can be plotted against the arc loading of the lamp, where arc loading is measured as the lamp power divided by the arc gap and where values typically fall in the range of between 60 and 120 watts per cm for metal halide discharge lamps. The power needed to achieve the number of lumens for this desired brightness level is determined by the efficacy of the lamp, which can be on the order of approximately 15 lumens per watt (lpw) for a xenon discharge lamp to approximately 70 or more lpw as in the present instance. At 75 lpw, to achieve 4500 lumens across a 2.7 mm arc gap, it would be necessary to operate the arc discharge at 60 watts as an example of an application of the present invention. In addition to the metal halide type of arc discharge described herein, it is known that xenon discharge lamps also provide a high brightness light output. Use of purely xenon discharge however, at approximately 15 lpw requires a significantly higher power rating to satisfy the lumens requirement and, in addition, the light output of a xenon discharge has a correlated color temperature (CCT) index of approximately 10,000.degree. Kelvin, which is significantly higher than the desired range for headlamp or general illumination purposes.
When considering brightness levels of the discharge lamp, it would be highly advantageous to achieve the desired lumen output at as low a power rating as possible thereby conserving energy and reducing the heat generated by the light source, heat which can adversely affect the optical fiber. In one example of a light source and reflector combination for use with optical fibers wherein a 6 mm arc gap is provided, the light output achieved is approximately 33,000 lumens per cm.sup.2 and is achieved using a 150 watt lamp. U.S. Pat. No. 5,016,152 issued to Awai et al on May 14, 1991 discloses such a light source disposed in an ellipsoidal reflector for focussing the light output to a focal point of the reflector. Though this patent discusses the desirability of increasing efficiency of light transfer to the optical fibers, there is no discussion of providing a light source having a high brightness level and a short arc gap thereby reducing the needed dimensions of the optical fibers.
It would be advantageous to a discharge light source having a short arc gap and high brightness output so as to be particularly suitable for use with optical fibers if such a light source exhibited long life characteristics where long life is typically considered to be on the order of 2000 hours of operation or longer. To obtain long life, it is known that a metal-halide light source must operate at a wall loading value of less than 20 watts per cm.sup.2. Therefore, for a high brightness light source particularly suited for operation with a light transmission arrangement such as optical fibers, it would be a significant advantage over existing light sources to provide a discharge lamp which achieves a relatively high brightness level using an arc gap substantially less than 4 mm in length, operates at a voltage which allows for high efficacy, requires a mercury density which results in an operating pressure well within the constraints of the mechanical properties of the arc tube and, which mercury density, along with the preferred arc tube dimensions, allows operation of the light source free from convective instability and also operates at a wall loading conducive to long lamp life.